1. Field of the Invention
The invention generally relates to fiber optic signal transmission systems and in particular to systems for transmitting both digital telephony and analog video signals.
2. Description of the Related Art
In many locations, optic fibers have been deployed for transmitting digital telephony signals, such as signals carrying telephone conversations, facsimile transmissions or Internet data communications. As shown in FIG. 1, an optic fiber 10 may interconnect a host digital terminal (HDT) 12 of a telephone company central office (CO) 14 with a curbside optical network unit (ONU) 16. The HDT provides an interface between the optic fiber and other components of the CO such as telephone switching equipment 18. The ONU provides an interface between the optic fiber and analog tip and ring telephone lines 22 connected into homes or offices 24. Usually only a single optic fiber is deployed between the CO and the ONU which carries both upstream digital telephony signals (i.e. signals sent from the ONU to the CO) and downstream digital telephony signals (i.e. signals sent from the CO to the ONU). Typically, the upstream and downstream signals are transmitted within separate transmission bands of the single optic fiber to avoid signal conflicts, crosstalk and the like. This is referred to as broad band wave division multiplexing. Optic fibers composed of silica have three useful transmission bands located at about 850, 1310 and 1550 nanometers (nm), which are hereinafter referred to respectively as the "850 band", the "1310-band" and the "1550-band". The existence of these bands is partly a function of the characteristics of the fiber itself, including such factors as the amount of optical absorption and dispersion within the fiber at different wavelengths, and partly a function of practical limitations on the availability of suitable devices, such as lasers and LED's, used for coupling light into the fiber at different wavelengths. As a result of these and other factors, it is currently most practical, at least for the purposes of digital telephony, to transmit either within the 1310-band or the 1550-band. The 850 band is not typically used for digital telephony.
In the example of FIG. 1, the downstream signals are transmitted into the 1310-band using an appropriate LED or laser configured for generating signals near 1310 nm. The upstream signals are transmitted into the 1550-band using an appropriate LED or laser for generating signals near 1550 nm. The transmission parameters and the operational characteristics of the fiber optic equipment are often configured to meet TA/R-909 loss budgets to assure reliable reception of signals despite losses associated with fiber splices and fiber connectors (not separately shown) and transmission losses in the fiber itself.
It is becoming increasingly desirable, however, to also provide for the transmission of other types of signals between the Co and the ONU along with the digital telephony signals. Specifically, it would be highly desirable to be able to transmit analog video signals, such as those provided by cable television (CAT) companies, from the CO to the ONU for subsequent routing into homes or offices. Indeed, by providing for the transmission of both digital telephony signals and analog video signals, the telephone company operating the optic fibers can thereby provide both telephone service and television service to its customers.
Problems, however, arise in connection with transmitting both upstream and downstream digital telephony signals as well as analog video signals over a single optic fiber. In particular, problems arise because the two aforementioned transmission bands, namely the 1550-band and the 1310-band, are the only two transmission bands that are commercially practical for transmitting digital telephony and analog video within silica fibers. Hence, only two transmission bands are available to handle the three required transmission channels, i.e. the upstream telephony, the downstream telephony and the downstream analog video.
One option is to transmit both the upstream and downstream telephony within common wavelengths of the 1310-band and to transmit the analog video within the 1550-band. This option is shown in FIG. 2, wherein downstream analog video, received from an analog video source 26, is transmitted by HDT 12 (or by a another device, such as a high density fiber bank (HDFN), not separately shown) over optic fiber 10 within the 1550-band to ONU 16 then converted to RF and transmitted through a co-axial cable 28 into houses or offices 24. Downstream telephony is transmitted over optic fiber 10 within the 1310-band to ONU 16 then converted to tip and ring signals and coupled into the houses or offices via tip and ring telephone lines 22. Upstream telephony is transmitted over optic fiber 10 within the 1310-band from ONU 16 to HDT 12 then converted to signals appropriate for coupling to switching equipment 18.
Thus, although not separately shown, the upstream end of the optic fiber is provided with an analog video 1550-band transmitter, a digital telephony 1310-band transmitter and a digital telephony 1310-band receiver. The downstream end of the fiber is provided with an analog video 1550-band receiver, a digital telephony 1310-band transmitter and a digital telephony 1310-band receiver. Appropriate couplers are employed for routing the telephony signals between the respective upstream and downstream 1310-band telephony transmitters and receivers and for routing the downstream analog video signals from the 1550-band analog video transmitter to the analog video receiver. In particular, a single-frequency coupler is employed at each end of the optic fiber for separating upstream and downstream telephony signals. The single-frequency coupler routes outgoing telephony signals onto the optic fiber from the respective transmitter and routes incoming telephony signals from the fiber into the respective receiver. A 1310/1550 window-splitting coupler (or, alternatively, a fused biconical tapered coupler (FBTC)) is also employed at each end of the optic fiber. The 1310/1550 window-splitting coupler at the upstream end of the optic fiber combines downstream telephony signals with downstream video signals for transmission over the optic fiber and splits off upstream telephony signals for routing to the upstream telephony receiver through the respective single-frequency coupler. The 1310/1550 window-splitting coupler at the downstream end of the optic fiber splits downstream telephony signals from downstream video signals for routing to the respective telephony or video receiver and couples upstream telephony signals onto the optic fiber.
However, the transmission of both upstream and downstream signals over the 1310-band through a single fiber leads to various problems. For example, "silent failure" can occur whereby a fracture in the optic fiber causes a transmitted signal to be reflected back along the optic fiber. In the example of FIG. 2, a digital telephony signal transmitted downstream in the 1310-band through the optic fiber may be reflected back upstream through the optic fiber as a result of a fracture (not shown). The 1310-band receiver at the upstream end of the fiber may erroneously receive the reflected signal and assume that the reflected signal was actually a signal transmitted from the downstream end of the fiber and that the connection to the downstream end of the optic fiber is still intact.
Silent failure can be detected by carefully managing optical power transmission levels and by determining whether all received signals lie within a narrow acceptable power level range consistent with a signal transmitted from the opposite end of the optic fiber. If a received signal has a power level that is too low or too high, it is presumed to be a reflected signal and appropriate error signals are generated. Alternatively, burst transmission schemes may be employed whereby the transmitter at one end of the optic fiber selectively transmits bursts of compressed data signals. The transmitter at the other end of the optic fiber transmits reply bursts after carefully timed intervals. If reply signals are received at some time other than within narrowly acceptable time intervals, the reply signals are presumed to be a reflected signals from a break in the optic fiber and appropriate action is taken. Although both techniques are capable of detecting silent failure, significant costs arise as a result of the need to either provide for careful power level management or to provide for burst processing.
Other problems also occur as a result of carrying both upstream and downstream digital telephony signals over the 1310-band. Crosstalk can occur between the transmitter and the receiver pair at each end of the fiber because both the transmitter and the receiver are operating in the same frequency band. Also, as noted, a single-frequency coupler is required at each end of the optic fiber to be able to carry both upstream and downstream telephony signals within the 1310-band. Single-frequency couplers typically cause a 3 db loss in signal power thereby reducing the overall efficiency of the system and hence adding associated costs.
Thus significant problems arise in attempting to carry both upstream and downstream digital telephony signals within common wavelengths of the 1310-band. Another single-fiber option would be to attempt to carry downstream digital telephony over the 1310-band and to carry both the upstream digital telephony and the downstream analog video over common wavelengths of the 1550-band. But many of the same problems as described above occur. Indeed, insofar as cross talk is concerned, matters are even worse because transmission power levels for analog video are typically far greater than for digital telephony so problems with cross talk are much more significant when downstream analog video is carried over the same transmission channel as upstream digital telephony, i.e. the upstream digital telephony receiver may erroneously receive a portion of the downstream analog video signal.
Moreover, significant difficulties arise when attempting to route downstream telephony over the 1310-band and to route both downstream video and upstream telephony over common wavelengths of the 1550-band when using conventional broad band couplers. Conventional couplers, such as 1310/1550 window-splitting beam-splitter couplers or FBTC's, are simply not effective for routing upstream 1550-band telephony signals over a single fiber to an upstream receiver while also routing downstream 1310-band telephony signals and 1550-band video signals over the same fiber to respective downstream receivers, at least not when common wavelengths of the 1550-band are employed for both the upstream telephony signals and the downstream video signals. In particular, such conventional couplers cannot be configured to adequately route upstream 1550-band signals onto the fiber while also splitting downstream 1310-band telephony signals from downstream 1550-band video signals received over the same fiber for coupling to separate receivers. Accordingly, with conventional systems, if video is to be transmitted along with telephony over a single fiber, the arrangement of FIG. 2 is employed wherein upstream and downstream telephony are both carried over the 1310-band and video is carried over the separate 1550-band. Although such an arrangement suffers from the problems summarized above, at least the necessary routing of the various signals from respective transmitters to respective receivers can be achieved using conventional couplers.
Yet another option, as shown in FIG. 3, is simply to provide a second optic fiber connecting the CO and the ONU with digital telephony carried over one fiber (10) and analog video carried over another (10'), but the cost of deploying a second optic fiber, particularly in areas already having a single optic fiber deployed, is usually prohibitive.
Accordingly, there is a significant need to provide for the ability to carry both downstream analog video and upstream and downstream digital telephony over a single optic fiber and it is to that end that the present invention are primarily directed.